Skeletal modeling represents the shaping of bone as the skeleton develops both pre- and post-natally in order to meet the needs of structural integrity and mechanical competence. Like remodeling, modeling requires coordination of osteoblasts (Obl), osteocytes (Ocy) and osteoclasts (Ocl). Unlike remodeling however, modeling osteoclasts and osteoblasts are not anatomically tethered but respectively resorb and form bone in locations which yield appropriate movement of the skeleton through space. During the tenure of this project, we demonstrated that bone forming cells communicate via gap junctions formed primarily by connexin43 (Cx43). The presence of cortical bone abnormalities in mice with Cx43 gene (Gja1) ablation in cells of the osteogenic lineage, and the fact that Gja1 mutations cause the human disease oculodentodigital dysplasia (ODDD), characterized by craniofacial and skeletal abnormalities, substantiate that skeletal homeostasis requires Cx43. In the last funding cycle, we made the unexpected observation that conditional Gja1 inactivation or induction of an ODDD Gja1 mutation in osteogenic cells primarily impairs cortical bone structure, leading to a larger total area, but decreased cortical thickness and compromised bone strength. This phenotype reflects increased endocortical resorption and periosteal apposition, as well as a hypomineralized and structurally abnormal bone matrix, associated with down-regulation of osteoprotegerin (Opg) and Sost in Obl/Ocy. Lack of Cx43 also alters the sensitivity of cortical bone to mechanical loading and unloading in an envelope-specific fashion. Hence, Cx43 in osteogenic cells controls envelope-specific formation and resorption of cortical bone, thus dictating its size, shape, biomechanical properties and responsiveness to mechanical stimuli. We therefore hypothesize that Cx43 is a key modulator of cortical bone modeling in the adult skeleton, and propose the following Specific Aims: 1) Mechanisms of Cx43 regulation of endocortical bone resorption, which will test the hypothesis that Cx43 regulates endocortical bone resorption via modulation of osteoprotegerin (Opg) expression by Obl and/or Ocy; 2) Mechanisms of Cx43 regulation of periosteal bone formation, which will test the hypothesis that Cx43 regulates periosteal bone formation, in part via modulation of Wnt signals; 3) Mechanical regulation of periosteal bone formation via Cx43, which will test the hypothesis that Cx43 modulates cortical bone responses to mechanical loading independently of bone architecture, via cell autonomous actions (Obl specific) and paracrine mechanisms (via the Ocy). Cortical bone (re)modeling is understudied but its importance in bone homeostasis and in the adaptive responses to mechanical factors is critical for maintaining bone strength and resistance to fractures. The proposed experiments will use genetic mouse models wherein Gja1 is selectively ablated or mutated at different stages of osteogenesis. Results will establish the mechanisms by which Cx43 modulates cortical bone and how such mechanisms cause the skeletal abnormalities of ODDD, findings that may be translated to clinical settings and lead to pharmacologic targeting.